The present invention relates to a circuit for generating a voltage, and more particularly relates to a voltage generator which generates a plurality of reference voltages using a resistor ladder and combines these reference voltages to generate a multistep output voltage.
A voltage generator for generating a multistep output voltage is used in various kinds of devices. For example, in a liquid crystal display, this type of voltage generator is used to generate a gray-scale voltage corresponding to a digital value indicating a luminance value, and the liquid crystal panel is driven using the generated gray-scale voltage. Image sensor systems also use voltage generators so as to convert an analog signal obtained by the image sensor circuit to a digital value indicating a luminance value. Other examples of devices that use such a voltage generator include digital-to-analog converters which convert a digital value to an analog signal, ramp generators which generate a ramp voltage whose voltage value increases stepwise, and analog-to-digital converters which convert an analog signal to a digital signal.
A conventional voltage generator includes a resistor ladder for generating a plurality of reference voltages, and a plurality of switches corresponding to a plurality of taps of the resistor ladder, and outputs, as the output voltage, one of the reference voltages by turning one of the switches on. In this way, the multistep output voltage is generated.
In this type of voltage generator, however, the number of reference voltages needs to be increased so as to increase the number of gray-scales of the output voltage. To that end, the number of resistance elements included in the resistor ladder and the number of switches must be increased, resulting in a larger circuit size. For example, 2048 (i.e., 211) resistance elements and 2048 switches are necessary to obtain an 11-bit output voltage, and 4096 (i.e., 212) resistance elements and 4096 switches are needed to obtain a 12-bit output voltage. In this manner, the greater the number of gray-scales of the output voltage, the larger the circuit size of the voltage generator.
In view of this, Japanese Patent No. 3611672 (Patent Document 1) discloses a technique for increasing the number of gray-scales of an output voltage without increasing the number of resistance elements and switches. In the voltage generator described in Patent Document 1, two switches are simultaneously turned on to generate an intermediate voltage, thereby increasing the number of gray-scales of the output voltage. Furthermore, if more switches are simultaneously turned on, the output voltage will have more gray scales.